1. Field of the Invention
This invention relates to a system for the artificial growing of dense populations of hatchery-derived filter-feeding shellfish such as free oysters and clams, with means provided for holding these populations in currents of nutrient-laden water. These means involve a flume system for growing the seed from hatchery sizes to intermediate sizes, with a special case for growing hardshell clams toward maturity in a flume, or "spillway" system. As the seed grows toward maturity, their requirements for current and for nutrients increase. A system is developed to provide these requirements. In this latter system, a progression of structures is utilized, beginning with a basic raft structure and adding channeling extensions to this primary structure. Each addition to the primary structure is designed to increase the efficiency of the system in utilizing a basic current and phytoplankton resource.
The system is designed to operate wherever there are currents and phytoplankton resources available. The system can be designed to be expanded to the limits of these resources.
2. Prior Art Relating to the Disclosure
Oysters, in their natural state, reproduce by spawning free swimming larvae that diffuse into the water medium. After a time for development, usually several weeks in duration, the larvae attach to suitable materials, usually clean, hard objects such as rocks and oyster shells. The natural home of the oyster is thus usually an oyster reef where oysters continually attach themselves to the shells of their predecessors, or a rocky beach or ledge. In these areas, the same currents that carry the oyster larvae carry the plankton to the attached oyster from which its food is derived.
The reproductive cycle of the oyster has been used to advantage by man by placing "cultch" materials -- shells, rocks, sticks etcetera on which the oyster larvae attach, and then cultivating the resulting seed. One of the earliest methods of this type cultivation was to plant the cultch with seed attached on suitable "bottom" areas of estuaries in populations dense enough to permit efficient care and harvesting. Using this type of culture seed must be both cheap and plentiful as losses in the growing population are high due to the actions of predators and silting. Further, the "bottom areas" are not the natural home for the oyster and feeding conditions are usually not as good as those in natural reefs and rocks where tide flows and currents are swifter. In the areas where oysters reproduce naturally, however, the success of the setting often results in overcrowding in the oyster population, both in terms of space and food.
To overcome the problems generated both by "natural" and bottom culture and to ensure better seed survival and oyster growth, more advanced types of cultures, utilizing various means to suspend the oyster off the bottom have been developed. These means include trays to hold the oysters, and include also stick or shells with seed attached. When shells are used they are generally spaced out on wires or ropes.
Recent developments in hatchery techniques have made possible the production of adequate supplies of oyster and clam seed for commercial operation regardless of natural sets. One of the results of hatchery technology has been the development of "cultchless" or "free" oyster seed where oyster seed is produced that is not attached to cultch. A method and apparatus for growing free oyster seed is described in U.S. Pat No. 3,517,648, wherein water containing nutrients is pumped continuously through the seed population carrying food to and detritus from the seed.
The major problem with the use of free seed has been to take it through from the tiny hatchery-produced stage to a size and condition where it has commercial value. Present hatchery technology limits the size to which the seed can be grown feasibly because of the costs involved in the production of feed for the seed.
The single oyster seed, while ideal from the standpoint of hatchery production, is virtually helpless in the natural sea bed environment of conventional bottom culture. Without its cultch as a platform and an anchor, the oyster falls easy victim to predator, to minor currents, and to siltation. Culture of the single oyster demands a nursery stage where the seed oyster can grow under protected conditions to a size where it can survive in efficient numbers in nature. Because space requirements for seed populations increase with the growth of the individual members of the population, economy demands that nursery techniques be devised which can grow the population at a level of concentration above that of a single layer. Unless such means can be found, the cost for providing space for the expanding populations to usable sizes makes such nursery operations prohibitive for, at least, the species of oyster experimented with by the applicant, the Crassostrea gigas, or Pacific oyster.